Download Today Newton`s Second Law of Motion Newton`s Third Law of

Common Exam 1
Feb. 27th, Wednesday
Time: To Be Announced
Room: To Be Announced
Check your NJIT e-mail and course website
HW due every Monday
Quiz every Thursday
Today
Newton’s Second Law of Motion
Newton’s Third Law of Motion
Forces
Gravity force (weight)
Normal force
1
Chapter 4
The Laws of Motion
Last class….
Newton’s First Law of Motion
If the net force on an object is zero,
‰ An object at rest remains at rest
‰ An object moving with constant velocity continues to move
with the same speed and in the same direction (the same
velocity).
In equation,
G
If Fnet = 0,
G
v does not change.
An additional force is NOT
required to keep a cart moving
with a constant speed on a
frictionless track!
2
What if the net force on an abject is NOT zero?
Æ Newton’s Second Law of Motion
Newton’s Second Law of Motion
• Unbalanced forces cause object to accelerate.
• An object of mass m has an acceleration a,
equal to the net force F, divided by the mass of
the object, m.
G
F
∑
G
F
G
a=
= net
m
m
3
G
∑F
G
G
Fnet
G
=
⇒ Fnet = ma
m
m
m
[1N ] = [1kg ] × 1 2 
 s 
G
a=
kg m
s2
SI unit of force is a Newton:
1N ≡ 1
Non-SI unit for force: pound (lb)
1 N = 0.225 lb
Newton’s 2nd law for each component
Fnet , x = ma x ; Fnet , y = ma y ; Fnet , z = ma z
Quick quiz
True or false? If the net force acting on an object
is in the positive x-direction, the object moves only
in the positive x-direction.
Answer: False
For example, consider projectile motion.
Example 1: Force and Acceleration Æ see note
4
Equal magnitude and opposite direction
Newton’s Third Law of Motion
• Forces always appear in pairs: for every force
exerted by object A on object B there is an equal and
opposite force exerted by B on A (action-reaction
pair).
• The 3rd law pair of forces are always applied to
different objects!
(Net force in 1st and 2nd laws is the sum of forces on
the same object.)
G
G
FA on B = − FB on A
5
Find reaction force for the gravity on the cantaloupe
Since the gravity is Force on the cantaloupe from the earth….
Answer: (Gravity) Force on the earth from the cantaloupe !
6
NOT the action-reaction pair!
Forces:
Gravitational Force:
¾
F = mg
Archimedes Force
¾
Friction Force:
¾
Tension Force
¾
Spring Force
¾
¾
Normal Force
7
Gravitational Force (Weight):
Weight: From the Earth on objects near Earth Surface
Magnitude:
F = mg, where g = 9.8 m/s2
Direction: Pointing downward
•Weight: The force that the Earth is pulling the
object with near the surface of the Earth.
•Weight is a vector quantity, it has a magnitude
and direction (unit : N)
(weight) = (mass) x g
Direction:
Magnitude:
Determined by analyzing forces
From surface on objects in contact
8
Example 2: Person in an elevator
Picture of Situation
A person with a mass 50 Kg in an elevator is moving upward
2
with an acceleration
a = 1.1 m / s
Identify all forces on the person and find magnitude and direction for each.
Example 3: Object on an inclined surface (see note)
9
Today, we learned
Newton’s Second Law of Motion
Newton’s Third Law of Motion
Forces
Gravity force (weight)
Normal force
10